1. Field of the Invention
The present invention relates to an illumination optical system, and more particularly, an illumination optical system used for a reticle alignment optical system of a projection exposure apparatus, and an alignment apparatus, and more particularly, an alignment apparatus suitable for reticle alignment in the projection exposure apparatus.
2. Related Background Art
In the alignment optical system of a projection exposure apparatus, when light supply itself must have some degree of freedom to illuminate a moving target object, e.g., when a transparent alignment mark provided on an index plate on a movable stage is to be illuminated to cause light to emerge therefrom, an illumination optical system having a light guide consisting of an optical fiber is used as an illumination optical system for illuminating the alignment mark. FIG. 1 is a view showing such a conventional illumination optical system using an optical fiber.
Referring to FIG. 1, a light beam radiated from a mercury lamp 300 serving as a light source is focused upon reflection by an elliptical mirror 301 and incident on an optical fiber 303 through a lens system 302. At this time, a light source image having a sufficient numerical aperture and diameter is formed at the incident end of the optical fiber 303. The light beam emerging from the optical fiber 303 forms an illumination field having an appropriate diameter and illumination numerical aperture through a condenser lens 304.
On the other hand, in manufacturing a semiconductor element or a liquid crystal display element, a projection exposure apparatus is used in which a reticle (or a photomask or the like) is illuminated with exposure light, and the pattern of the reticle is focused and projected onto a wafer (or a glass plate or the like) coated with a photoresist through a projection optical system. Conventionally, continuously emitted light such as a beam from a mercury lamp (e.g., the g-ray or the i-ray) is used as the exposure light. In such a projection exposure apparatus, the reticle and wafer must be accurately aligned with each other in the exposure operation. For this purpose, first of all, the reticle is aligned with respect to a reticle stage by using a measurement result from a reticle alignment system.
Upon completion of reticle alignment, the position of the reticle is measured with reference to a wafer stage on which the wafer is mounted. The wafer is aligned on the basis of the measurement result. To measure the positional relationship of the reticle with respect to the wafer stage, a reticle position detection system of a so-called stage light emission type is used in some cases.